Carbon Monoxide

Oh boy. The rocket mass heater we have built has just been so difficult. After the ´wet wood´(not) dilemma we rebuilt it, but we are still having huge problems. We are committed to making it work.... for personal pride and to justify the work we have already put into it.... especially my husband. Just yesterday, in desperation, my husband installed a ventilation fan. Still set off the CO alarm.

So I was wondering if anyone out there could give us some more advice.. You guys are so great for sharing your knowledge and skills, thanks in advance . We are having awful problems with carbon monoxide entering the room.. usually at about the time the fire is dying down.  Which of these things is most likely to be the culprit ? In other words, which one would you start with ?

1)I read in a blog that, ´The exhaust vent of your stove must be at least 3 feet above your roof in order to reduce the risk of CO being drawn back into your structure. ´ Ours isn´t.
2) We haven´t gone with the oil drum ( I can just hear the gasps of horror...), but rather with a brick column around the heat riser. Could this be leaking carbon monoxide from gaps in the fire cement?
3) We haven´t gone with the cob bench but rather a stratification chamber. Could this be leaking CO from gaps in the fire cement?
4) The vermiculite clay layer around the heat riser is really thick. Could it still be wet after about a dozen burnings ? Could this be causing the fire to not burn cold enough ? Even so would this cause the CO to back into the room?
5) Anything else ?

I just want to add, that with this current rebuild we followed Ianto Evans measurements from his classic rocket mass heater booklet very closely...except for the comments above.

We would really appreciate any thoughts on this.
Warmly
Linda

A rmh should be running with a draft while it's still warm, in other words at least while you have combustion occurring. Is there a draft?

Graham, Thanks for your help. My husband´s answer is, Ýes, but we don´t know how effectively.´ Oh, and I just saw on a YouTube from Honey Do Carpenter that if you´re exhaust tube isn´t insulated the draft of cold air from outside will push the gases back into the house....more questions... sorry

Well, there are lots of different causes of back drafting if that is the case.   People use matches, tissues etc to see if there is a draft working.  So, when your CO alarm sounds, check to see if the draft is there or not.

Hi Linda,
1) It will take a skilled mason to build a brick bell which is gas tight. I would start with plastering the bricks in order to seal all the small leaks.
2) You say the chimney isn't above the roof line: at which level is it? Some people think a single-wall chimney pipe would do as the outside chimney, please believe me, it won't. Requirements for a good chimney: 3 feet above roof line, straight, smooth inside and insulated.
3) And yes, the vermiculite layer could still be wet since it tends to stock moisture in a very efficient way. A dozen firings won't remove that in my experience, keep firing that rocket ferociously.

Solving point 1 and 2 is absolutely necessary in my opinion.

Hi Linda, the ideal situation for a wood burning stove is that there be a good strong natural draft even when the stove is stone cold. A strong natural draft helps keep the stove under negative pressure at all times, pulling any residual combustion gasses up the chimney and out of the dwelling. With the proper chimney, even a freshly built, wet, stone cold RMH will exhibit a good natural draft. This is of even greater importance for stoves of "single skin" construction.

What's your chimney like? Is the exterior (outdoors portion) of the chimney insulated? Does the chimney exit at a height of 3 to 4 feet above the tallest point of the roof? For us mortals that don't reside in a magical valley with consistent unidirectional winds (re Ianto Evans) there's just no getting around having a proper chimney attached to our RMH.

Thanks to all, Peter, Graham and Byron for your input. One more question.  The flue pipe, the one that connects the strat chamber with the outside world is very wide. It will be cheaper for us to insulate and extend this pipe if it is narrower. Are there any guidelines or restrictions on the diameter of that tube? That should keep us busy for a while.....

Hi Linda,

Ideally the exhaust / flue pipe diameter should match RMH system size, i.e. 6 inch inside diameter for 6 inch RMH, the stove's system size being defined by the size of the wood feed-tube opening. What size system did you all build?

BTW, only the portions of the flue that are outdoors and in un-heated attic spaces need be insulated.

Hi Linda;

Peters sugestion of plastering over your brick work is the FIRST thing to do.   Especially around your brick riser!
 After that you need to extend your chimney pipe taller.  Do you need to insulate that pipe ouside the roof ???  Mine in northern Montana is uninsulated, but it does stick up above the roof line.  You are in Spain, I could be wrong having never been there, but I think it is a temperate climate?  Insulating your pipe outside the house (not needed indoors) would be nice but I do not believe it is the cause of your CO2 readings.
Plastering your bricks well, I believe is your answer.  OR if you have many free brick then a second skin(using concrete not clay)  around your first bricks would seal it up , same as a masonry heater.

In the US, masonry heaters are required to be built with double skins for just that reason, to avoid CO leakage in a single skin. Plaster or cob on the outside will give the same effect.

Chimney pipe should be 2 feet taller than anything within 10 feet of it. That is what i was taught decades ago.

Linda, I think your getting some very helpful replies, with many suggesting the second skin/plastering.  It also sounds like your doing this work yourself.  I would like to suggest, that the second skin if done with bricks, GAINS much from the joints not being in the same place as your inner wall of bricks.  Bricks are dimensional  i.e. 4.5 x 9 or 1/2 tall as they are long.  This makes 1/2 lapping a fairly easy task by either starting with a 1/2 brick, or turning a horizontal brick vertical. Corners can be ended opposite of the inner layer-  All the while your laying "this second layer" look to where the joints will come out compared to the inner layer.  

You will find, if using the excellent fire brick mortar available, that making a very thin but smooth joint is possible.  With the slightest touch of water, you can almost get plaster smooth to SEAL everything. You can do this inside and out.  i.e. on your first layer outside, joints of the second layer, and second layer outside. YOU should not have to take anything apart if you feel your first core was done correctly.

Perhaps I missed it, but I don't see where you say, that your fire roars to life at any point in the burn..  Does it?  If not, then perhaps this leads us into another area? design build flaw? I have found that sometimes the slightest error in the build can cause a lackluster burn.

Just thoughts.  

Linda, since you're getting CO alarms at end of the burn, the primary problem to fix is the draft.  Even metal stoves with no air leaks can do this if the draft reverses.
Can we have pictures of all your pipe work and angles?  And where is the carbon monoxide detector installed?  Near the ceiling?

Thanks everyone! One more question, can the flue pipe that exists the strategy chamber be  smaller than the system is? Byron, I know you said it needed to be, but were you referring to the standard RMH with cobbed in pipes in bench? I mean isn't the strategy chamber itself like a pipe?
After consideration of all this lovely information we're going to do the following
1) Plaster the stratification chamber on the inside, then across the top
2) Plaster the outside of the brick column in place of the oil drum
3)Extend the flue pipe to 3 ft above the roof and use double skinned insulated pipe for the outside section. This will be necessary as although we live in Spain, we chose an altitude of 2,700 ft, so winters are wintery.

We'll let you know how it goes.....maybe we'll be able to give you all a mental high five!!
Blessings,
Linda

Correction ....the pipe that EXITS the stratification chamber....

The short reply is no it shouldn't be smaller.
But …  How much smaller ? How tall is your chimney ?  Were you burning this before with the smaller exhaust stack ?
The smaller chimney could be the cause of your CO2 problem...

More info. please

EDIT) I see, before it was larger and now you are thinking of changing it to smaller.   Same size as your feed tube. 6" feed tube 6" chimney.